Workshop Exercises: Advanced Chemical State Analysis

1) Carbon 1s - Set up a standard file for the fitting of adventitious carbon. Download the C 1s spectrum here.

2) Use it to charge correct a series of spectra. Download test file.

3) Oxygen 1s - General fitting of the O 1s spectra for metals.  Use file from number 2. Fit the O 1s spectrum, understand and explain all sources of oxygen.

4) Ti 2p - Set up curve-fitting parameters.  Download Ti 2p test file. See titanium literature fitting values.

5) Cr 2p - Set up curve-fitting parameters. Download Cr 2p test file. See chromium literature fitting values.

Notes:
•Cr(VI) Species – mix of oxide and hydrated species
   -One narrow peak FWHM 1.5 eV
  -Range from 579.0 to 580.0 eV
•Cr(III) Species – mix of oxide and hydrated species
  -Cr(OH)3 - One broad peak FWHM of ~2.5 eV, set to 577.5 eV
  -Cr2O3 - Five multiplet peaks of equal FWHM (~0.9 eV) with set areas and separations based on standard sample
•Cr(0) – Metal
  -One asymmetric peak with a FWHM of 0.9 eV
  -Range from 573.9 to 574.5 eV

6) Ni 2p - For the brave, download and give it a try. See Ni 2p literature fitting values and general instructions here.

What is Adventitious Carbon?

A thin layer of carbonaceous material is usually found on the surface of most air exposed samples, this layer is generally known as adventitious carbon. Even small exposures to atmosphere can produce these films. Adventitious carbon is generally comprised of a variety of (relatively short chain [1]) hydrocarbons species with small amounts of both singly and doubly bound oxygen functionality. The source of this carbon has been debated over the years. It does not appear to be graphitic in nature and in most modern high vacuum systems vacuum oils are not readily present (as they have been in the past) [1,2,3,4]. There may be some evidence that CO or CO2 species may play a role in the gradual appearance of carbon on pristine surfaces within the vacuum of the XPS chamber [3].

It’s presence on insulating surfaces provides for a convenient charge reference by setting the main line of the C 1s spectrum to 284.8 eV (although values ranging from 285.0 eV to 284.5 eV have been used in some cases, remember to check for this value when looking for binding energy references in the literature). The error in this value (284.8 eV) is, for most systems, on the order of +/-0.2 eV to 0.3 eV.  An in-depth look at the effectiveness of using AdC for charge correction purposes, including standardized fitting procedures, is presented in [5].

  

Work by Grey et al. [6] has explored the nature of adventitious carbon by XPS and time-of-flight secondary ion mass spectrometry (ToF-SIMS).  XPS D-parameter and ToF-SIMS analyses confirms that AdC is not graphitic in nature. An average C 1s spectrum for AdC (Figure 1, Table 1) was derived and shows that, on average, ~ 25 % of the carbon species in AdC is directly associated with oxygen functionality.  Similarly, ToF-SIMS analyses show that AdC is comprised of mainly short chain hydrocarbons with some oxygen functionality.

An advanced method for curve-fitting of the C 1s envelope for AdC (Table 2) was developed that included the effects of beta carbons (in this context, the alpha carbon is the carbon directly attached to the oxygen, and the beta carbon is attached to the alpha carbon) and were informed by the configurations of possible volatile organic compounds (VOC) that are the source of most AdC [6]. Using this method in combination with the dataset from [5], the average C–C/C–H AdC aliphatic peak position was shown to be 284.81 eV (+/- 0.25 eV) via verification with a secondary internal reference.

Figure 1. Average of 80 adventitious carbon C 1s XPS spectra.

Table 1. Average adventitious carbon C 1s fitting parameters from an average of 80 AdC spectra.

Table 2. Curve-fitting parameters for AdC C 1s including shifted beta peaks (*) (peaks E, F and G). Areas for peaks A, B, C, and D should be left unconstrained. # If peak-shape for peak D is well-defined the FWHM constraint can be removed.

References:

[1] T.L. Barr, S. Seal, J. Vac. Sci. Technol. A 13(3) (1995) 1239.

[2] P. Swift, Surf. Interface Anal. 4 (1982) 47.
[3] D.J. Miller, M.C. Biesinger, N.S. McIntyre, Surf. Interface Anal. 33 (2002) 299.
[4] H. Piao, N.S. McIntyre, Surf. Interface Anal. 33 (2002) 591.

[5] M.C. Biesinger, Appl. Surf. Sci, 597 (2022) 153681.

[6] L.H. Grey, H.-Y. Nie, M.C. Biesinger, Appl. Surf. Sci. 653 (2024) 159319.

Using Adventitious Carbon for Charge Correcting

The C 1s spectrum for adventitious carbon can be fit as follows.  A single peak, ascribed to alkyl type carbon (C-C, C-H), is fit to the main peak of the C 1s spectrum.  A second peak is added that is constrained to be 1.5 eV above the main peak, of equal FWHM to the main peak (C-C, C-H). This higher binding energy peak is ascribed to alcohol and/or ester functionality (C-OH, C-O-C). Further high binding energy components can be added if required. For example: C=O at approximately 3 eV above the main peak and O-C=O at 3.8 to 4.3 eV above the main peak. One or both of these peaks may also have to be constrained to the FWHM of the main peak if they are poorly resolved.  Reference [1] and the table below outline standard starting fitting parameters for adventitious carbon. 

Adventitious carbon C 1s curve-fitting parameters [1].

Spectra from insulating samples can then be charge corrected by shifting all peaks to the adventitious C 1s spectral component (C-C, C-H) binding energy set to 284.8 eV. There is certainly error associated with this assignment. Swift [2] lists a number of studies showing errors ranging from ±0.1eV to ±0.4 eV.  “Newer” studies (late 1970's) range from ±0.1 to ±0.3 eV. “Older” studies (late 1960's to early 1970's) were in the ±0.4eV range - however, reproducibility and resolution of the spectrometers of the time may have played a role.  Barr's [3] work from 1995 states that error in using adventitious carbon is ±0.2 eV.  Our work [4] in 2002 also suggests error in the ±0.2eV to  ±0.3eV range.  Experience with numerous conducting samples (1995 to present) and a routinely calibrated instrument have shown that the C 1s signal generally ranges from 284.7 eV to as high as 285.2 eV [5].  Reference [1] presents a detailed assessment of the analysis of insulating samples from a multi-user facility from over a 5-year period that showed an adventitious C 1s (C-C, C-H) binding of 284.91 eV ±0.25eV.  A similar study confirming the utility of the adventitious carbon technique with a similar multi-user facility analysis has been published by Morgan [6].

For organic systems, especially polymers, it is convenient to charge correct to the C-C, C-H signal set to 285.0 eV. This makes for easier comparison to the polymer handbook [7] which uses this number for charge correction.

References:
[1] M.C. Biesinger, Appl. Surf. Sci, 597 (2022) 153681.

[2] T.L. Barr, S. Seal, J. Vac. Sci. Technol. A 13(3) (1995) 1239.
[3] P. Swift, Surf. Interface Anal. 4 (1982) 47.
[4] D.J. Miller, M.C. Biesinger, N.S. McIntyre, Surf. Interface Anal. 33 (2002) 299.
[5] M.C. Biesinger, unpublished results

[6] D.J. Morgan, Surf. Interface Anal. 57 (2025) 28.
[7] G. Beamson, D. Briggs, High Resolution XPS of Organic Polymers - The Scienta ESCA300 Database Wiley Interscience, 1992.

Graphitic/Graphene/Carbon Nanotube C 1s Curve-Fitting

Materials of a graphitic nature (e.g., graphite, graphene, carbon nanotubes etc.) will have a C 1s main peak, attributed to C=C, which can be used as a charge reference set to 284.5 eV. An average of values for graphite from 21 references from the NIST database [1] is 284.46 eV with a standard deviation of 0.14 eV. Note that the well characterized value of 284.5 eV for graphitic carbon is also a strong indicator that this value is not appropriate as a value to use for AdC charge referencing. While these types of samples are generally conductive and if they can be mounted in a manor (in electrical contact with the sample stage) to take advantage of this one should do so. However, many of these types of samples come as a small volume of powders or flakes which are very difficult to mount. Usually, we mount these on a double-sided adhesive which works well but electrically isolates the sample. Oxidation of these types of samples (e.g., graphene oxide) or their functionalization (e.g., functionalized CNTs) can result in them behaving less conductively or as a mixed conductive/insulating material.  Samples where these materials are mixed with other conducting or insulating compounds can also result in a mixed conductive/insulating sample. For most of these types of samples we now electrically isolate the sample and charge reference to C 1s at 284.5 eV for the graphitic (C=C) peak.[2]

Table 1 from [2] presents general fitting parameters for graphitic, graphene and carbon nanotube type materials. These starting fitting parameters include the main peak asymmetry (defined using an asymmetric Lorentzian (LA) line shape) and π to π* shake-up satellite from a pure graphite standard sample. These fitting parameters are similar to the approach taken by Morgan (Fig. 5, Table 2) [3],  Moeini et al. (Table 1) [4],  and Gengenbach et al.[5]  It is always best to run your own standard (pure graphite, graphene, CNT etc.) to get fitting parameters appropriate for your sample type, instrument and conditions used. Slight differences in the main peak asymmetry and differing shake-up satellite position, shape and intensities are possible for differing classes of graphitic materials. See for example from Morgan[3] where HOPG and nano-onion C 1s spectra show peak-shape differences, likely due to hydrogenation of the sample. However, with this caveat stated, the parameters used based on a graphite standard have worked very well for variety of samples (134) analyzed in the five-year data survey from [2]. Figure 1(A) presents the standard graphite spectrum used to obtain the parameters presented in Table 1. The spectra from Figure 1(B, C and D) show the use of these fitting parameters from Table 1 to effectively model a variety of graphitic component containing materials. 

Table 1. General fitting parameters for graphitic/graphene/carbon nanotube type materials. #Line-shape details for CasaXPS. Define asymmetric peak-shape in other software using pure graphite/graphene or CNT sample related your specimens. ##Gaussian/Lorentzian product formula, GL(30) is 30% Lorentzian 70% Gaussian.[2]

Figure 1.  Examples of curve-fitting of graphitic type systems using the parameters from Table 1.  A) pure graphite, B) carbon nanotube-based material modified in caustic solution, C) oxidized graphene and D) acid modified graphene and organic compound mixture.[2]

References:
[1] C.D. Wagner, A.V. Naumkin, A. Kraut-Vass, J.W. Allison, C.J. Powell, J.R.Jr. Rumble, NIST Standard Reference Database 20, Version 3.4 (web version) (http:/srdata.nist.gov/xps/) 2003.
[2] M.C. Biesinger, Appl. Surf. Sci. 597 (2022) 153681.
[3] D.J. Morgan, J. Carbon. Res. 7 (2021) 51.
[4] B. Moeini, M.R. Linford, N. Fairley, A. Barlow, P. Cumpson, D. Morgan, V. Fernandez, J. Baltrusaitis. Surf. Interface Anal. 54 (2022) 67.
[5] T.R. Gengenbach, G.H. Major, M.R. Linford, C.D. Easton, J. Vac. Sci. Technol. A, 39 (2021) 013204.

Systematic and Collaborative Approach to Problem Solving using X-ray Photoelectron Spectroscopy

Our recent article [1] in Applied Surface Science Advances highlights methodology developed as a result of years of interactions between many junior and senior X-ray Photoelectron Spectroscopy (XPS) users operating within the CasaXPS spectral processing and interpretation program framework. In particular, discussions arising from a series of workshops have been a significant source for developing the overall XPS data processing concept and are the motivation for creating this work. These workshops organized by the Institut des Matériaux Jean Rouxel (IMN), Nantes gather both experienced and novice users of XPS for a week of discourse in conceptual experiment design and the resulting data processing. However, the framework constructed and utilized within these workshops encouraged the dissemination of knowledge beyond XPS data analysis and emphasized the importance of a multi-disciplinary collaborative approach to surface analysis problem-solving. The material presented here embodies data treatment originating from data made available to the first CNRS Thematic Workshop presented at Roscoff 2013. The methodology described here has evolved over the subsequent workshops in 2016 and 2019 and currently represents the philosophy used in CasaXPS spectral data processing paradigm.

This article also serves as a useful reference descriptor of the CasaXPS software program. 

Reference:

[1] N. Fairley, V. Fernandez, M. Richard‐Plouet, C. Guillot-Deudon, J.Walton, E. Smith, D. Flahaut, M. Greiner, M. Biesinger, S. Tougaard, D. Morgan, J. Baltrusaitis, Systematic and collaborative approach to problem solving using X-ray photoelectron spectroscopy, Applied Surface Science Advances, 5 (2021) 100112.

Workshop Exercises: The Auger Parameter and Wagner Plots

The Auger Parameter

1. Try calculating the Auger parameter for ZnO and Cd metal (click on links to download the spectra). (Compare to zinc and cadmium literature.)
2. Calculate the Auger parameter and use it to determine the chemical state for 3 unknown Cu species (Unknown 1, Unknown 2, Unknown 3).  Click here to access Cu binding energy and Auger parameter values.

Wagner Plots
Handouts will be given or you can download the exercise here.

Use lines of slope 1 and 3 to look at the trends:
1) For the Ga(III) halogen series.
2) Going from Ga(III), Ga(II) to Ga(I).
3) For the metal, alloys and semiconductors (Ga2O3 is also a semiconductor).
Do final state effects dominate (slope of 3) or do initial state effects dominate (slope of 1)?

Workshop Exercises: Survey Scans

1) Download exercise 1 file - follow along...
2) Download exercise 2 file.  See link to quantification from survey scans.

XPS Details Needed for Publications

Below is a list of some of the XPS experimental details that may be needed for most scientific publications. Beside each detail are some examples of what could be used or stated.

1) Instrument Name: Kratos Axis Supra, Kratos Axis Nova, Surface Science Laboratories SSX-100, PHI Quantera, PHI VersaProbe, VG ESCALAB 250, type of energy analyzer etc.

2) X-ray Source: monochromatic Al K(alpha) at 1486.6 eV, non-monochromatic Mg K(alpha), Zr L, He(I), He(II), synchrotron tunable source set at ‘x’ kV

3) X-ray Power: 15 mA and 15 kV, 225 W (= 15mA x 15 kV)

4) Spectrometer Calibration Details: The instrument work function was calibrated to give a binding energy (BE) of 83.96 eV for the Au 4f7/2 line for metallic gold and the spectrometer dispersion was adjusted to give a BE of 932.62 eV for the Cu 2p3/2 line of metallic copper.

5) Spot Size, Area of Analysis, Angle: 300x700 micron area of analysis, 300 micron spot, small spot analysis using a 55 micron spot size, 90 degree take-off angle / 0 degree of sample tilt, angle resolved analysis

6) Charge Neutralizer Use: The Kratos magnetic confinement charge compensation system was used on all samples (One may also want to include charge neutralizer settings although this can vary depending on the age of the filament, contamination of the charge plates etc. A note on the how good charge neutralization was deemed to have occurred may suffice). An electron flood gun at ‘x’ settings.

7) Effectiveness of Neutralizer: Charge neutralization was deemed to have been fully achieved by monitoring the C 1s signal for adventitious carbon. A sharp main peak with no lower binding energy structure is generally expected.

8) Charge Correction Procedures: Spectra have been charge corrected to the main line of the carbon 1s spectrum (adventitious carbon) set to 284.8 eV (or 285.0 eV for polymer samples), Au 4f7/2 at 83.96 eV, no charge correction (conductive samples)

9) Instrument Base Pressure: 8 x 10^-10 Torr.

10) Scan Details, Pass Energy, Number of Sweeps, Step Size, Scan Window: The C 1s spectra were taken with a minimum of 10 - 60s scans with a scan window of 278-295 eV using a 0.05 eV step and 20 eV pass energy.

11) Spectrometer Resolution Details: Ag 3d5/2 line FWHM at 10 eV pass energy was 0.48 eV. Source resolution for monochromatic Al K(alpha) X-rays is ~0.3 eV.  The instrumental resolution was determined to be 0.35 eV at 10 eV pass energy using the Fermi edge of the valence band for metallic silver. Resolution with charge compensation system on <0.68 eV FWHM on PET.

12) Sample Mounting Details: held by metal clips and grounded to the holder, mounted on double sided adhesive tape, electrically isolated from the sample holder 

13) Software Used for Curve-Fitting: CasaXPS version (2.3.26), XPSPeak, Vision 2 Processing Software, Avantage

14) Line-shape Details: 50% Gaussian/50% Lorentzian, Asymmetric line-shape defined by…,

15) Background Used for Curve-Fitting: Shirley, Linear, Tougaard

16) Other Curve-Fitting Details as Needed: FWHM, constraints, doublet separations

A recent article from Pinder et al.[1] discusses a similar list as above for reporting instrument parameters. They also discuss common errors seen in XPS publications, well worth a read as a quality check prior to publishing your own data. 

Reference:

[1] J.W. Pinder, G.H. Major, D.R. Baer, J. Terry, J.E. Whitten, J. Cechal, J.D. Crossman, A.J. Lizarbe, S. Jafari, C.D. Easton, J.Baltrusaitis, M.A. van Spronse, M.R. Linford, Appl. Surf. Sci. Adv., 19 (2024) 100534.

Sulphur

Table 1. S 2p3/2 binding energies compiled from the NIST database [1] and other sources.

Notes: 2p3/2 - 2p1/2 doublet separation = 1.18eV, peaks constrained to a 2:1 area ratio (2p3/2 : 2p1/2), generally one sets both peaks to an equal FWHM for ease of use although in pure samples this may not be the case.

Smart et al. [9] and Pratt et al. [10] give an excellent overview of binding energy ranges for the study of mineral surfaces. These ranges can be used with other sulphur containing systems as well. Of particular interest is the assignment for polysulphides eg. (S4)2- = 162.0-163.0 eV, (S5)2- = 161.9 - 163.2 eV, (Sx)2-) = 163.7 eV. Surface species can also play a role in XPS, especially for in-situ fractured sulphide mineral species [11].

[a] Nesbitt et al. [12] give a value of 162.2 eV for the disulphide in arsenopyrite.

[b] A more detailed look at organic sulphur species can be found here.

In a recent paper from Sarah Harmer's group at Flinders University, synchrotron XPS is used to convincingly elucidate surface 3-coordinate, bulk and surface 4-coordinate and bulk 5-coordinate sulfur in the chalcogenide (Fe,Ni)9S8.  This work shows that sulfide coordination changes can be seen by XPS [13].

A Na2S2O3.5H2O (sodium thiosulphite cooled to -130C during analysis) reference sample gave S 2p3/2 peak positions of 162.1 eV and 168.1 eV for S*SO3 and SS*O3 moieties, respectively.

There is a lot of confusion in the literature when presenting the data for sulphur. Some papers mention S 2p when they really mean S 2p3/2, these are not interchangeable! Please remember to be specific about the exact peak you are referring to.  

A recent paper from Clark et al. [14] highlights how widespread the issue of erroneous peak fitting of S 2p is.  Section B within this paper is worth a look as it highlights some of the common errors that should be avoided, these include:

1) Lack of spin–orbit splitting. Doublets (2p3/2 and 2p1/2 peaks) in their appropriate 2:1 ratios, respectively, should be used to represent each chemical state in the material.

2) Inconsistent and widely varying peak widths/full widths at half maximum (FWHMs).

3) Questionable assignments of the peaks to chemical species or oxidation states.

4) Backgrounds that cut through and then extend above the data on the high and low binding energy sides of the peak envelopes. 

5) Relatively large range of peak binding energy positions or fit components that are assigned as the same chemical states and should have well defined positions.

6) Noisy spectra, insufficient S/N.

References:

[1] C.D. Wagner, A.V. Naumkin, A. Kraut-Vass, J.W. Allison, C.J. Powell, J.R.Jr. Rumble, NIST Standard Reference Database 20, Version 3.4 (web version) (http:/srdata.nist.gov/xps/) 2003.
[2] Z.E. Pettifer, J.S. Quinton, S.L. Harmer, Minerals Engineering, 184 (2022) 107666.

[3] A.N. Buckley, W.M. Skinner, S.L. Harmer, A. Pring, L-J. Fan, Geochimica et Cosmochimica Acta, 73 (2009) 4452-4467.

[4] A.N. Buckley, W.M. Skinner, S.L. Harmer, A. Pring, R.N. Lamb, L.J. Fan, Y. Yang, Canadian Journal of Chemistry, 85 (2007) 767- 781.

[5] S.L. Harmer, A.R. Pratt, H.W. Nesbitt, M.E. Fleet, Canadian Mineralogist, 43 (5) (2005) 1619-1630.

[6] M.E. Fleet, X. Liu, S.L. Harmer, H.W. Nesbitt, Surface Science, 584 (2005) 133-145.

[7] V.P. Zakaznova-Iakovleva, S.L. Harmer, H.W. Nesbitt, G.M. Bancroft, A.R. Pratt, R. Flemming, Surface Science, 600(2) (2006) 348-356.

[8] A.R. Pratt, H.W. Nesbitt, American Mineralogist, 85 (2000) 619-622.

[9] R.St.C. Smart, W.M. Skinner and A.R. Gerson, Surface and Interface Analysis, 28 (1999) 101-105.

[10] A.R. Pratt, I.J. Muir and H.W. Nesbitt, Geochimica et Cosmochimica Acta, 58 (2) (1994) 827-841.

[11] H.W. Nesbitt, M. Scaini, H. Hochst, G.M. Bancroft, A.G. Schaufuss and R. Szargan, American Mineralogist, 85 (2000) 850-857.

[12] H.W. Nesbitt, I.J. Muir, A.R. Pratt, Geochimica et Cosmochimica Acta, 59 (9) (1995) 1773-1786.

[13] Z.E. Pettifer, J.S. Quinton, W.M. Skinner, S.L. Harmer, Applied Surface Science, 504 (2020) 144458. 

[14] B.M. Clark, G.H. Major, J.W. Pinder, D.E. Austin, D.R. Baer, M.C. Biesinger, C.D. Easton, S.L. Harmer, A. Herrera-Gomez, A.E. Hughes, W.M. Skinner, M.R. Linford, Journal of Vacuum Science and Technology A, 42 (2024) 063213.

The Chemical Significance of XPS BE Shifts: A Perspective

A recent publication from Paul Bagus (University of North Texas), Connie Nelin, and Dick Brundle [1] discusses the chemical significance of XPS BE shifts. Paul, Connie and Dick have made many outstanding contributions to the field of XPS, in particular by using computational chemistry approaches to model various XPS phenomena and spectral shapes - especially of transition metal species with complex multiplet splitting and satellite structures. 

Dr. Bagus describes this perspective below. This will be a good starting point for researchers interested in applying MO theory to XPS measurements. 

An all too common interpretation of the shifts of XPS BEs, Delta BE, for a given element in different compounds and in different environments is to relate the sign of the BE shift to the change in the effective charge, Q, of the core ionized atom. Thus, a shift to lower BE from sample 1 to sample 2 is interpreted as meaning that the atom in sample 2 has a smaller positive Q or a larger negative Q than the same atom in sample 1. Similarly, a shift to a larger BE is taken to mean that the atom in sample 2 has a larger Q. This paper shows that this simple interpretation of BE shifts is incomplete and that it is likely to be misleading.

While the effective charge Q does contribute to BE shifts, it is not the only physical or chemical mechanism that can contribute to XPS BE shifts. Two other mechanisms are the environment of the ionized atom that can lead to electrostatic potential that are different at different sites in a given sample and are different for different samples. Another mechanism is the degree of hybridization of an atom again at different sites and different compounds. An important objective of this perspective is to examine the mechanisms that lead to BE shifts. The chemical and physical content of these different mechanisms is first examined for a model system. With this model system, the different mechanisms can be separated and the magnitudes of the XPS BE shifts due to the different mechanisms can be understood directly in terms of the electronic charge distribution. Then five specific examples of XPS BEs measured for real systems are discussed and the observed BEs related to the physical and chemical mechanisms which are the origin of the BE shifts. The paper also considers the initial and final state contributions to the BE shifts and identifies when it is likely that initial state effects will dominate.

An important goal of the paper is that the principles and mechanisms for BE shifts can be applied, not only to the specific systems discussed in the paper but also to the understanding of the Delta BE for systems in general. It can lead readers to make suggestions for theoretical studies to help explain specific observations of BE shifts.

Reference:

[1] P.S. Bagus, C.J. Nelin, C.R. Brundle, J. Vac. Sci. Technol. A 41 068501 (2023).